One of the major problems facing industries such as mining, precious metals, and energy industries (e.g., coal mining and coal fired power plants) is removal of selenium and arsenic from process effluent to meet federal and state compliance standards.
Stringent standards for the maximum level of pollutants in water to be used for drinking or related to ground water systems are being promulgated by federal and state agencies. The current allowable maximum concentration level for selenium in drinking water set by federal standards is 0.01 milligrams per liter. Selenium removal from ground water presents a challenge in certain geographical areas of the United States. The State of New Mexico has proposed a selenium standard of 0.05 milligrams per liter for discharge into the ground water system of the state.
The presence of arsenic in water beyond the permissible limit (0.05 mg/l) has carcinogenic effects in living beings. The toxicity of arsenic varies greatly according to its oxidation state. As(III) has been reported to be more toxic than As(V) and methylated arsenic compounds. In a study of subsurface migration of organic and inorganic arsenic in an abandoned herbicide manufacturing plant, a total arsenic concentration of 900 mg/l was recorded at a depth of 35 feet of sand underlain with a 50 foot clay layer.
Several methods are available for reducing selenium and arsenic concentrations to acceptable levels in aqueous solutions. One method employed to remove or substantially reduce the concentration of soluble inorganic pollutants such as heavy metals in water is chemical precipitation of the metals as their oxides or their hydroxides. This precipitation generally is effected by the addition of lime, alum or an iron salt to the water at an appropriate pH. Other treatment methods, such as ion exchange, reverse osmosis, electrolysis or distillation, also can be effective in removing various pollutants. However, these methods are considerably more expensive and generally narrower in application scope than is desirable for the treatment of great volumes of water. Often it is difficult to remove trace selenium and arsenic using the conventional methods.
It is known that selenium ions can be removed from aqueous systems employing chemical precipitation if the selenium is present in the selenite (Se(IV)) oxidation state. Generally, such precipitation methods comprise treating the selenium-containing aqueous system with an iron salt, such as ferric or ferrous sulfate, chloride or hydroxide, or with aluminum or zinc in some appropriate form such as powder or granules. However, such chemical precipitation methods provide only very limited removal of selenium when it is present in the selenate (Se(VI)) oxidation state. Therefore, when selenium is present in the selenate oxidation state, its removal generally is effected by either ion exchange or reverse osmosis.
In particular, experimental studies have shown that chemical precipitation employing ferric sulfate can achieve a significant removal of selenium in the selenite oxidation state from an aqueous stream. More particularly, when ground water containing 0.03 milligrams per liter of selenium in the Se(IV) oxidation state and having pH of 5.5 is treated with 30 milligrams of ferric sulfate per liter, about 85% of the selenium is removed from the water (U.S. Environmental Protection Agency, "Manual of Treatment Techniques for Meeting the Interim Primary Drinking Water Regulations," May 1977, pages 29-31).
U.S. Pat. No. 3,933,635 discloses a process for removing selenium ions present in the selenite oxidation state from acidic process waters. Acidic process water, having a pH of about 1.0 to 4.0, is reacted with a metallic reducing agent at a temperature in the range of from about 25.degree. C. to about 85.degree. C. for a sufficient time to reduce the soluble selenium in the selenite oxidation state to insoluble elemental selenium. Preferably, the temperature is maintained in the range of from about 50.degree. C. to about 70.degree. C. The reducing agent can comprise aluminum, iron or zinc in an appropriate form, such as, for example, a powder, scrap fragments, granules, wools and the like. The preferred reducing agent for selenium in the selenite oxidation state is zinc powder.
In contrast, laboratory tests and pilot plant studies have shown that chemical precipitation, employing alum, lime, ferrous sulfate or ferric sulfate, is substantially ineffective for removing selenium in the selenate (Se(VI)) oxidation state from water. Studies on water having a selenium Se(VI) concentration of 0.03 to 10 milligrams per liter have shown that the conventional chemical precipitation methods remove less than 10 percent of the selenium from the water (U.S. Environmental Protection Agency, "Manual of Treatment Techniques for Meeting the Interim Primary Drinking Water Regulations," May 1977, pages 29-31).
It has been shown that selenium ions in the selenate oxidation state can be removed by ion exchange or reverse osmosis. As previously indicated, these methods are prohibitively expensive when large volumes of an aqueous solution must be treated. Further, both methods produce a contaminated regeneration effluent that requires further treatment for selenium fixation or removal before disposal. Thus, the selenium removal problem still exists but in a more highly concentrated solution.
U.S. Pat. No. 4,405,464 provides a method by which selenium, as the selenate, can be chemically precipitated from an aqueous system using metallic iron. This process is also disclosed as being capable of removing a substantial portion of any selenium in its selenite oxidation state. This process is economically more attractive than either the ion exchange or reverse osmosis methods which have been proposed or which are currently in use. However, this method is not suitable for aqueous solutions having pH greater than 6.0. Thus, if the water is alkaline or neutral it is preferably acidified through the addition of a quantity of a mineral acid such as, for example, hydrochloric acid or sulfuric acid or any other acidic solution such as acidic mill process water.
Literature published to date by the EPA and others reports removal efficiency of greater than 98% of selenium with ALCOA.TM. F-1 alumina. However, the high chemical impurity level in F-1 results in a low point of zero charge which means low specific adsorption of negatively charged ions. F-1 alumina also has an inherently low chemisorption capacity, which makes this material a poor choice as an adsorbent for selenium in any form.
The following publications provide background information on the removal of selenium and arsenic from aqueous solutions:
1. Baldwin, R. A. et al., "Removal and Recovery of Selenium from Aqueous Solutions," U.S. Pat. No. 4,519,913 (1985). PA0 2. Baldwin, R. A. et al., "Process for the Removal of Selenium from Aqueous Solutions," U.S. Pat. No. 4,405,464 (1983). PA0 3. Bar-Yosef, B. and Meek, D., "Selenium Sorption by Kaolinite and Montmorillonite," Soil Science (1987) 144:11. PA0 4. Brierley, C. L., "Selenium in Mine and Mill Environments," Randol at Mine Expo '92, p. 175. PA0 5. Ghosh, M. M. and Yuan, J. R., "Adsorption of Inorganic and Organoarsenicals on Hydrous Oxides," Environmental Progress (1987) 6(3):150-157. PA0 6. Gupta, S. K. and Chen, K. Y., "Arsenic Removal by Adsorption," Journal of Water Pollution Control Federation (March 1978) 50(3):493. PA0 7. Harper, T. R. and Kingham, N. W., "Removal of Arsenic from Wastewater Using Chemical Precipitation Methods," Water Environmental Research (1992) 64(3):200-203. PA0 8. Jinadasa, K. B. P. N. and Dissanayake, C. B., "The effect of selenium on fluoride-clay interactions: possible environmental health implications," Environmental Geochemistry and Health (1992) 14(1):3-7. PA0 9. Marchant, W. N., "Method for Removing Soluble Selenium from Acidic Wastewater," U.S. Pat. No. 3,933,635 (1976). PA0 10. Masscheleyn, P. H. et al., "Transformations of Selenium as Affected by Sediment Oxidation-Reduction Potential and pH," Environ. Sci. Technol (1990) 24(1):91-96. PA0 11. Merrill, D. T. et al., "Field Evaluation of Arsenic and Selenium Removal by Iron Coprecipitation," Environmental Progress (1987) 6(2):82-90. PA0 12. Murphy, A. P., "Removal of Selenate from Water by Chemical Reduction," Industrial Engineering and Chemical Research (1988) 27:187-191. PA0 13. Rubel, F. R. and Hathaway, S. W., "Pilot Study for Removal of Arsenic from Drinking Water at the Fallon, Nevada, Naval Air Station," Project Summary, Research and Development, EPA/600/S2-85/094, September 1985. PA0 14. Fitzgerald, N. M. et al., "Novel Magnesium Oxide-Aluminum Oxide Hydrotalcite-like Adsorbent for the Removal of Toxic Metals from Aqueous Stream," Alcoa Laboratories, Alcoa Center Pa.